Optical regenerator and an optical transmission system

ABSTRACT

A a first clock extraction device included in the receiving section of the optical regenerator recovers a receiving clock. The receiving clock follows the jitter contained in a receiving optical signal caused by fiber wavelength dispersion. The receiving optical signal is detected in synchrony with the receiving clock. A transmission section includes a transmitter for generating a transmission optical signal changed in phase in such a manner as to suppress the stimulated Brillouin scattering and a second clock extraction device for regenerating the transmission clock not following the jitter. The data of the receiving optical signal is processed at a processing unit, and intensity-modulated on the basis of the data thus processed. The timing of this modulation is synchronized with the transmission clock. The receiving clock is synchronized with the jitter, and therefore no logic error occurs. Since the transmission clock does not follow the jitter, the jitter is prevented from accumulating.

BACKGROUND OF THE INVENTION

The present invention relates to an optical regenerator and an opticaltransmission system for multi-repeater optical transmission, or more inparticular to a high-quality optical transmission technique employing ameasure against what is called the stimulated Brillouin scattering inorder to produce a high launch power.

In an optical transmission system, implementation of practical opticalamplifiers has made it possible to produce a high-output opticaltransmission signal from an optical transmitter. The power of theoptical transmission signal that can be applied to an optical fibercoupled to the repeater, however, is limited by the stimulated Brillouinscattering (hereinafter referred to simply as "SBS") of the opticalfiber.

A method for improving the power of the optical transmission signal thatcan be applied to the optical fiber is described in ECOC Journal,pp.657-660, (1991) by P. M. Gabla, et al.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptical transmission system that can transmit an optical transmissionsignal with high power.

Another object of the invention is to provide an optical regeneratorsuitable for the optical transmission system repeated by a plurality ofregenerators.

According to P. M. Gabla, et al., SBS can be suppressed by modulatingthe phase or frequency of the signal input to the optical fiber to havean uncontinuous phase characteristic. In the case where the input signalto an end of the optical fiber, i.e., the frequency of the output signalof a repeater is different, however, the time of arrival of the signalat the other end of the optical fiber is diverged due to the wavelengthdispersion of the optical fiber, with the result that a jitter appearsin the signal waveform at the other end.

Detailed description will be made with reference to FIG. 1. In FIG. 1,reference numeral 1 designates an anti-SBS transmitter including asemiconductor laser 9 as a CW light source which emits at 1.3 μm or 1.5μm drived by a predetermined magnitude of current. Numeral 8 designatesa amplitude modulator for frequency-modulating with a sinusoidal wave(several kHz to several MHz) the predetermined current applied to thesemiconductor laser. As a result, the output phase or frequency of thesemiconductor laser 9 is modulated, thereby generating a carrier signalwhich has an uncontinuous phase capable of suppressing SBS. In theprocess, the output amplitude of the semiconductor laser is alsomodulated. This amplitude modulation, however, is negligibly small. Theamplitude of this carrier signal is modulated in accordance with thedata signal by an external modulator 10 and makes up an opticaltransmission signal. When a signal with a discrete phase is applied toan optical fiber 11, the output waveform of the optical fiber contains ajitter for the reason described above. The observed frequency of thejitter coincides with the frequency of the frequency modulator 8.

As shown in FIG. 2, the frequency modulator 8 may be eliminated and thephase of the output of the laser 9 may be modulated directly using aphase modulator 100 to suppress the jitter.

Repeaters 20 and 30 which have been studied by the inventors are shownin FIGS. 3A and 3B.

In these repeaters, a received signal containing the jitter is logicallydecided at an edge of the clock pulse extracted from the received signaland then transmitted as an optical signal (by regeneration repeating).The modulation frequency of the anti-SBS frequency modulation is assumedto be fSBS, and the cut-off frequency of the jitter transfer functionfor a clock extraction device to be fC1. The configuration with therelation fSBS<fC1 held is shown in FIG. 3A, the jitter transfer functionfor the clock extraction device 4' in FIG. 4A. Since the jitterfrequency fSBS of the receive signal is lower than the cut-off frequencyof the jitter transfer function, the clock phase extracted by the clockextraction device 4' follows the jitter of the data. As a consequence,the data phase coincides with the clock phase at a decision block 5, andtherefore no logical error of the data is caused due to the jitter. Inspite of this, the jitter on input side passes through the opticalregenerator 20 and appears in a similar form in the optical intensitywaveform on the output side. The frequency modulation for SBSsuppression is required to be performed for each optical regenerator.After transmission through the optical fiber 11, therefore, thefrequency modulation is converted into a jitter by wavelengthdispersion, resulting in an increased jitter. In this way, when fSBS issmaller than fC1, the problem is that the jitter is accumulated duringthe multi-repeater transmission.

Now, the configuration for the relation fSBS>fC1 held is shown in FIG.3B, and the jitter transfer function for the clock extraction device 4"in FIG. 4B. In view of the fact that the input jitter frequency fSBS ishigher than the cut-off frequency of the jitter transfer function, theclock extraction device 4" suppresses the jitter of frequency fSBS. Nojitter by anti-SBS frequency modulation, therefore, appears on theoutput. Since the clock extracted the clock extraction device 4" failsto follow the data jitter, therefore, an error occurs between the clockphase and the data phase at the decision block 5, which is likely tocause a logic error of the data.

In these regenerators, the transmission quality is deteriorated due tothe jitter caused when the SBS is suppressed.

Accordingly, it is an object of the present invention to provide anoptical regenerator and an optical transmission system in which thelogical error and the jitter accumulation due to the received jitter canbe obviated in the presence of jitters due to the phase or frequencymodulation performed for the SBS suppression.

In order to achieve the above-mentioned object, there is providedaccording to the invention an optical regenerator in which an opticaltransmission signal frequency-modulated by which phase of the signalchanges to suppress the SBS and further intensity-modulated by data istransmitted as an optical output, the optical regenerator having twofunctions of clock extraction for decision of the received signal andclock regeneration for a transmission clock. In other words, as shown inFIG. 5, there is provided an optical regenerator comprising a firstclock extraction device 4' for decision of the received signal and asecond clock extraction device 4" for regenerating the transmissionclock, wherein the relation holds that fC2<fSBS<fC1, where fSBS is themodulation frequency for frequency modulation of the opticaltransmission signal, fC1 is the cut-off frequency of the jitter transferfunction of the first clock extraction device 4', and fC2 is the cut-offfrequency of the jitter transfer function for the second clockextraction device 4".

In this optical regenerator, suppose the first clock extraction device4' and the second clock extraction device 4" are cascaded as shown inFIG. 5. The first clock extraction device 4' follows the input and thesecond clock attraction device 4" can easily suppress the jitter throughthe extraction devices 4'. This configuration is desirable as the jittertolerance and the jitter transfer characteristic can be improvedsimultaneously.

Also, in order to achieve the above-mentioned objects of the invention,there is provided according to the present invention a multi-repeateroptical transmission system for transmitting an optical transmissionsignal frequency-modulated and further intensity-modulated by data as anoptical output, in which the optical transmission signal of thetransmitter is frequency-modulated with frequency fSBS, a correspondingrepeating receiver includes a first clock extraction device 4' fordecision the received signal and a second clock extraction device 4" forregenerating the transmission clock as a clock for transmission data, asshown in FIG. 5, and the relation is held that fC2<fSBS<fC1 where fC1 isthe cut-off frequency of the jitter transfer function for the firstclock extraction device and fC2 the cut-off frequency of the jittertransfer function for the second clock extraction device.

Even when a jitter occurs in the received signal due to the frequencymodulation for SBS suppression, the relation fSBS<fC1 causes the phaseof the input clock to the decision block to follow the phase of theinput data and therefore no error occurs. In view of the fact that thetransmission clock holds the relation fC2<fSBS, on the other hand, thefrequency component of jitter for anti-SBS frequency modulation issuppressed, and therefore jitters are not added independently oraccumulated at the time of multiple repeating operations. In this way,the present invention is provided to solve the two problems of jitterand the logical error due to the input jitter at the same time.

In the foregoing description, assume that the anti-SBS transmitter shownin FIG. 2 is used. In the case where the phase uncontinuity of thetransmission signal is converted to the instantaneous frequency or inthe case where the modulation by the frequency modulator 8 is not bysinusoidal wave, as shown in FIGS. 4C and 4D, the instantaneousfrequency corresponding to the modulation frequency fSBS has a certainbandwidth. In this case, the relations hold that fC2<fSBSL andfSBSH<fC1, where fC1 is the cut-off frequency of the jitter transferfunction of the first clock extraction device 4' and fC2 is the cut-offfrequency of the jitter transfer function of the second clock extractiondevice 4".

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and technical advantages of the presentinvention will be readily apparent from the following description of thepreferred exemplary embodiments of the invention in conjunction with theaccompanying drawings, in which:

FIGS. 1 and 2 are schematic diagrams showing an anti-SBS transmitter andjitter generation;

FIGS. 3A and 3B are diagrams showing the configuration of a repeaterstudied by the inventors;

FIGS. 4A to 4D are diagrams showing the jitter transfer characteristicof the clock extraction devices;

FIG. 5 is a diagram showing the configuration of an optical regeneratoraccording to a first embodiment of the invention; and

FIG. 6 is a diagram showing the configuration of an optical regeneratoraccording to a second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detail below.

An example configuration of an optical regenerator according to a firstembodiment is shown in FIG. 5. Numeral 40 designates an opticalregenerator unit, numeral 2 a photodiode, numeral 3 an equalizingamplifier, and numeral 4' a first clock extraction device. The clockextraction device, which includes such functions as nonlinearextraction, is represented by a tank circuit related to the jittercharacteristic. Numeral 5 designates a decision block 9 numeral 6 adigital signal processing section for data processing required forregenerator, numeral 7 a D-FF (D-type flip-flop), and numeral 4" asecond clock extraction device. The clock supply for driving the digitalprocessing section 6 is not shown as it is not important in terms ofconfiguration. The D-FF 7, which indicates that the phase (timing) ofthe output data is determined by the clock supplied from the clockextraction device 4", may be included in the digital processing section6 depending on the particular configuration. Numeral 8 designates anamplitude modulation oscillator for frequency modulation suppressingSBS, numeral 9 a semiconductor laser, and numeral 10 an externalmodulator.

First, the operation at the transmitting end will be explained. When thecurrent supplied to the semiconductor laser 9 is modulated by anoscillator 8 of a frequency fSBS in order to reduce SBS, the lightspectrum of the output of the semiconductor laser is diverged. Thislight is modulated by data through the external modulator 10 thereby toproduce an optical signal intensity-modulated according to the data.

Next, the operation at the receiving end will be explained. The opticalsignal received through the dispersive optical fiber contains a jitteraccording to the principle shown in FIG. 1. This received optical signalis converted into a current by the photodiode 2, amplified by theequalizing amplifier 3, and then is distributed between the decisionblock 5 and the first clock extraction device 4'. The first clockextraction device 4', which generally includes a nonlinear extractor,for example, full-wave rectifier, a tank circuit and a limittingamplifier, may alternatively use a PLL. Provided, however, that fSBS<fC1where fC1 is the cut-off frequency of the jitter transfer function. Inthe case where a tank circuit is used for the clock extraction device4', Q1 is determined as Q1=fB/(2fC1), where fB is the transmission bitrate. In actual transmission systems, it is necessary that 500<Q1<1500,and fSBS is required to be determined after fC1 has been determined forvarious reasons. The clock recovered at the clock extraction device 4'is supplied as a timing signal for logic decision on the equalizingamplification signal to the decision block 5 on the one hand and isapplied to the second clock extraction device 4" at the same time. Giventhe relation that fSBS<fC1, the jitter of frequency fSBS passes throughthe clock extraction device 4'. As a result, the clock supplied to theclock extraction device 4' follows the jitter of the equalizingamplification signal applied to the decision block 5. There occurs,therefore, no logic error of data which otherwise might be caused by thejitter generated for SBS suppression.

The clock extraction device 4" is for removing the jitter of the clockrecovered at the clock extraction device 4', and includes a tank circuitalone or a PLL. The clock extraction device 4" is input the clockincluding the jitter of frequency fSBS, and in order to suppress thisjitter, the relation is held fC2<fSBS. The transmission data is outputsyncronized with the clock regenerated at the clock extraction device 4"using the D-FF 7. As a consequence, the jitter of frequency fSBS inputto the optical repeater 40 is removed from the output. Also, no logicerror of data occurs which otherwise might be caused by the jitter offrequency fSBS.

As will be seen from the foregoing embodiment, the object of theinvention is achieved by establishing the relation fC2<fSBS<fC1, wherefSBS is the frequency-modulating frequency fSBS for SBS suppression, fC1is the cut-off frequency of the jitter transfer function for the firstclock extraction device, and fC2 is the cut-off frequency of the jittertransfer function for the second clock extraction device. The differencebetween fC1 and fSBS, which may be small, is preferably about one orderof magnitude considering a margin. Also, the difference between fSBS andfC2 should preferably be one order of magnitude. For example, accordingto the embodiment under consideration, fSBS is set to 10 kHz to 1 MHz,fC1 to 3 MHz to 10 MHz, and fC2 to 1 kHz to 0.1 MHz for 10 Gb/stransmission.

Unlike in the aforementioned embodiment in which the first clockextraction device and the second clock extraction device are connectedin cascade, parallel connection of the first and second clock extractiondevices has a similar same effect. Also, according to theabove-mentioned embodiment, the semiconductor laser 9 for SBSsuppression is frequency-modulated with a sinusoidal wave. A similareffect is obtained, however, by the frequency modulation with other thana sinusoidal wave or by the phase modulation instead of frequencymodulation as far as fC2 is set to less than the lower limit (fSBSL) ofthe spectrum of the modulation signal and fC1 to more than the upperlimit (fSBSH) of the spectrum of the modulation signal.

When a multiplicity of repeaters as described with reference to theforegoing embodiment are connected, jitters will neither accumulate norcause a logic error. As a result, a transmission system comprising suchrepeaters can realize high-quality optical transmission between remotesites.

A repeater 50 according to another embodiment for 10 Gb/s opticalregenerate is shown in FIG. 6. In FIG. 6, the component parts whichperform the same operation as the corresponding parts in FIG. 5 aredesignated by the same reference numerals respectively and will not befully described. A first clock extraction device 54', like the firstclock extraction device 4' of FIG. 5, is for recovering the clock inorder not to cause any logic error due to jitters. The first clockextraction device 54' includes a full-wave rectifier 61, a tank circuit62 and a limitting amplifier 63. The transmission optical signal isconverted into a first electrical signal at a photodiode 2, which firstelectrical signal is amplified at an amplifier 3 and is distributedbetween the first clock extraction device 54' and the decision block 5.The first clock extraction device 54' recovers the clock signal 80 (10GHz) on the basis of the first electrical signal distributed. The clocksignal 80 is divided to 1/16 in frequency by a demultiplexer 51, andafter being further reduced to 155 MHz by a frequency divider circuitnot shown and built in a receive data processing section 52, is sent toa first PLL circuit 71 as a second clock signal 81. The first PLLcircuit 71 is a filter for removing the jitter contained in the secondclock signal. A third clock 82 free of the jitter is sent to a secondPLL circuit 72 and further to a transmission data processing section 55.The first electrical signal identified at the decision block 5 isparallely converted in frequency by the demultiplexer 51 and an internalfrequency dividing circuit into a second electrical signal. This secondelectrical signal is processed at an arithmetic circuit of theprocessing section 52. The clock signal 82 free of jitters is increasedin frequency from 155 MHz up to 10 GHz by the second PLL circuit 72 (toform a clock signal 83). The transmission data processing section 55operates at a timing of the 155 MHz clock signal 84. The frequency ofthe transmission data rate is increased up to 10 GHz by a multiplexer 56and a frequency multipher circuit not shown and built in the processingsection 55. The transmission data is applied to a modulator driver 57 ata timing of the clock signal 83 by means of the D-FF 7.

An elastic buffer 53 arbitrates between the phase of writing data atCLOCK 81 including the jitter and the phose of readiry data at CLOCK 84including nogitter. As a result, data are positively delivered from thereceiver 90 to the transmitter 91.

As far as the second PLL circuit 72 is provided with the function ofremoving jitters, the first PLL circuit 71 is done without. According tothe embodiment under consideration, the cut-off frequency fC1 of thetank circuit 62 is set to 3 MHz to 10 MHz, and the cut-off frequency fC2of loop filter for the first PLL circuit 71 to 1 kHz. The cut-offfrequency of loop filter the second PLL circuit 72 could not be reducedbelow 3 MHz for the reason of reducing random jitter. Consequently, thefirst PLL circuit 71 is required.

The operation of the processing sections 52 and 55 is described in U.S.patent application Ser. No. 08/044,425 filed Apr. 7, 1993 whichapplication is incorporated herein by reference.

The repeater 50 according to the present embodiment, like the repeater40 in FIG. 5, effectively prevents the logic error and the jitteraccumulation even when a jitter is contained in the received signal.

We claim:
 1. An optical regeneration repeater comprising:means forreceiving an optical signal from a different optical regenerationrepeater or transmitter, said means regenerating data from a receivedsaid optical signal; means for generating a transmission optical carrierhaving an uncontinuous phase in such a manner so as to suppress astimulated Brillouin scattering; means for modulating said transmissionoptical carrier on a basis of the data regenerated by said means forreceiving; means for transmitting said modulated optical signal; firstclock extraction means for recovering a first clock from said opticalsignal, said first clock incorporating a jitter; and second clockextraction means for generating from said optical signal a second clockfor giving a timing for said transmission optical carrier, said secondclock not incorporating said jitter.
 2. An optical regeneration repeateraccording to claim 1,wherein said optical regeneration repeater operatesaccording to a relationship of fC2<fSBS<fC1, where fSBS is a frequencycorresponding to the uncontinuous phase change of the received saidoptical signal, fC1 is a cutoff frequency of a jitter transfer functionof said first clock extraction means, and fC2 is a cut-off frequency ofa jitter transfer function of said second clock extraction means.
 3. Anoptical regeneration repeater according to claim 1,wherein said secondclock extraction means recovers said second clock from said first clock.4. An optical regeneration repeater according to claim 2,wherein saidfSBS has a bandwidth, and said optical regeneration repeater operatesaccording to a relationship of fC2<fSBSL and fSBSH<fC1, where fSBSH andfSBSL are a maximum value and minimum value of the bandwidth,respectively.
 5. An optical regeneration repeater according to claim1,wherein said means for generating a transmission optical carrierincludes a semiconductor laser providing a source of CW oscillation andan amplitude modulator for modulating a current applied to saidsemiconductor laser.
 6. An optical regeneration repeater according toclaim 1,wherein said means for generating the transmission opticalcarrier includes a semiconductor laser providing a CW oscillation sourceoscillating at a predetermined frequency and a phase modulator forphase-modulating an output light of said semiconductor laser.
 7. Anoptical regeneration repeater according to claim 2,wherein said firstclock extraction means includes a tank circuit.
 8. An opticalregeneration repeater according to claim 7,wherein said first clockextraction means further includes a full-wave rectifier and a limittingamplifier.
 9. An optical regeneration repeater according to claim2,wherein said first clock extraction means includes a PLL circuit. 10.An optical regeneration repeater according to claim 2,wherein saidsecond clock extraction means includes a tank circuit.
 11. An opticalregeneration repeater according to claim 2,wherein said second clockextraction means includes a PLL circuit.
 12. An optical regenerationrepeater according to claim 2,wherein said second clock extraction meansincludes at least two PLL circuits coupled in series.
 13. An opticalregeneration repeater according to claim 2,further comprising means forforming a third clock having a frequency which is different from saidfirst clock, wherein said second clock extraction means generates saidsecond clock from said third clock.
 14. An optical transmission systemcomprising a plurality of regeneration repeater according to claim 1.15. An optical transmission system according to claim 14, furthercomprising a repeater amplifier between said opt.
 16. A method ofoptical regenerating comprising the steps of:receiving an input opticalsignal containing a jitter, and regenerating data from said inputoptical signal using a first clock extracted from said input opticalsignal and containing said jitter; intensity-modulating a transmissionoptical signal having an uncontinuous phase in such a manner as tosuppress a Brillouin scattering, in synchronism with a second clockwhich is independent of said jitter, and on a basis of the dataregenerated in said receiving step.
 17. An optical regeneratorcomprising:a photoelectric converter for receiving an input opticalsignal containing a jitter and having an uncontinuous phase in such amanner as to suppress a stimulated Brillouin scattering, and forming afirst electrical signal from a received said input optical signal; atank circuit for regenerating a first clock signal for generating saidfirst electrical signal from said input optical signal, said first clocksignal containing said jitter; processing means for processing saidfirst electrical signal and forming transmission data; a second tankcircuit regenerating a second clock signal on a basis of said firstclock signal, said second clock signal not containing said jitter; atransmission signal generator for generating a transmission opticalsignal having an uncontinuous phase in such a manner as to suppress astimulated Brillouin scattering; and a modulator forintensity-modulating said transmission optical signal on a basis of saidtransmission data in synchronism with said second clock signal.
 18. Anoptical transmission system comprising a plurality of opticalregenerates according to claim
 17. 19. An optical transmission systemaccording to claim 18, further comprising a repeater amplifier betweenthe optical regenerators.
 20. An optical regenerator comprising:aphotoelectric converter for receiving an input optical signal containinga jitter and having an uncontinuous phase in such a manner as tosuppress a stimulated Brillouin scattering, and forming a firstelectrical signal from a received said input optical signal; a firstclock extraction circuit for regenerating a first clock signal forgenerating said first electrical signal from said input optical signal,said first clock signal containing said jitter; means for reducing afrequency of said first clock signal and a frequency of said firstelectrical signal thereby to form a second clock signal and a secondelectrical signal; first processing means for processing said secondelectrical signal in synchronism with said second clock signal andforming a processed data; a first PLL circuit for removing the jitterfrom said second clock signal so as to generate a third clock signal; asecond PLL circuit for generating a transmission clock signal from saidthird clock signal; second processing means for forming transmission damfrom said processed data in synchronism with said third clock signal; anelastic buffer for synchronizing a transfer of said receiving data fromsaid first processing means to said second processing means; means forincreasing a frequency of said transmission data; a transmission signalgenerator for generating a transmission optical signal having anuncontinuous phase in such a manner as to suppress a stimulatedBrillouin scattering; and a modulator for intensity-modulating saidtransmission optical signal on a basis of said transmission data insynchronism with said transmission clock signal.
 21. An opticaltransmission system comprising a plurality of optical regeneratorsaccording to claim
 20. 22. An optical transmission system according toclaim 21, further comprising a repeater amplifier between said opticalregenerators.